    const arr = [2, 4, undefined, 9, 7];
    console.log("1==============");
    for (let i = 0; i < arr.length; i++) {
        const ele = arr[i];
        if (ele === undefined)
            console.log("1: ", i);
    }

    console.log("2==============");
    arr.forEach(item => {
        if (item === undefined)
            console.log("2: ", item);
    })

    console.log("3==============");
    for (const key in arr) {
        if (arr[key] === undefined)
            console.log("3: ", key);
    }

    console.log("4==============");
    for (const [index, iterator] of arr.entries()) {
        if (iterator === null)
            console.log(index);
    }